The one-way clutch is disposed between, e.g., an annular outer race and an annular inner race so disposed concentrically with the outer race as to be spaced inwardly of the outer race, and is configured to include a plurality of sprags hindering the outer race and the inner race from making a relative rotation in a predetermined direction by being engaged with an inner peripheral surface of the outer race and an outer peripheral surface of the inner race, retainers that retain the sprags, and spring that gives biasing forces to the sprags in such a direction as to be brought into engagement with the outer race and the inner race. With this configuration, the one-way clutch serves as a torque transfer mechanism, in which the inner race and the outer race rotate on idle relatively in one direction about a central axis but transfer a rotational torque in a reversed direction.
Exemplified as a one-way clutch used as a speed change mechanism in an automatic transmission for an automobile is a one-way clutch including annular end bearings having a bearing function in order to keep concentricity between the outer race and the inner race (e.g., Patent Document 1).
The one-way clutch using the end bearings is effective in decreasing lubricity for ensuring a lubrication oil, but produces a lubrication oil film in a peripheral direction on a bearing surface between the outer/inner races and the end bearings. The production of this lubrication oil film leads to a problem of causing a drag torque when rotating on idle as a viscous resistance of the oil. The drag torque when rotating on idle is part of a drive resistance of the automobile, and is an issue to be reduced by a clutch mechanism in terms of improving fuel consumption of the automobile and a contribution to the environment concerning CO2 emission control.
Such being the case, there is made a proposal (e.g., Patent Document 2) for reducing the drag torque when rotating on idle by changing members exhibiting the bearing function to block bearings from the end bearings to thereby improve a shearing property of the lubrication oil film but decrease the viscous resistance owing to intermittent production of the lubrication oil film in the peripheral direction.